1. Field of the Invention
The present invention relates to a laminated filter, integrated device, and communication apparatus which are used in high-frequency radio equipment such as a cellular telephone.
2. Related Art of the Invention
In recent years, owing to a continued reduction in size of communication equipment, dielectric laminated filters, which are effective in size reduction, have often been used as high frequency filters. An example of such a laminated band-pass filter will be described with reference to the drawings.
FIG. 4 is a sectional view of a conventional laminated band-pass filter 400. In this figure, the laminated band-pass filter 400, of thickness d, formed by laminating and integrating a plurality of dielectric layers together, is sandwiched between internal grounding conductors 401 and 402, and a first band-pass filter 404 is formed on a top surface of the internal grounding conductor 401, with an internal grounding conductor 403 arranged on the first band-pass filter and a second band-pass filter 405 formed on the internal grounding conductor 403. Strip line conductors 410 are provided substantially in the center of each of the first and second band-pass filters 404 and 405 in a thickness direction thereof.
The first and second band-pass filters 404 and 405 are laminated together while being shielded by the internal grounding conductor 403, thereby reducing the interference between the two filters.
However, in the above described configuration, the interval between the internal grounding conductors sandwiching the corresponding one of the first and second band-pass filters 404 and 405 therebetween (i.e. the interval between the internal grounding conductors 402 and 403 and the interval between the internal grounding conductors 401 and 403) is smaller than that between interval between internal grounding conductors for a single band-pass filter 501 (see FIG. 5) formed inside an integrated device (i.e. the interval between internal grounding conductors 511 and 512).
For comparison, the sectional view in FIG. 5 shows a conventional laminated band-pass filter 500 having the band-pass filter 501.
In FIG. 5, the single band-pass filter 501, formed inside an integrated device having a thickness d, is sandwiched between the internal grounding conductors 511 and 512. Strip line conductors 510 (see FIG. 5) are provided substantially in the center of the band-pass filter 501 in the thickness direction thereof.
The inventors have found that in the first and second band-pass filters 404 and 405 (see FIG. 4), the Q factor of the strip line conductors, which constitute the band-pass filter, may decrease to increase an insertion loss to this device compared to the band-pass filter 501 (see FIG. 5).
More specifically, as shown in FIG. 10 illustrating simulation-based analysis of the behavior of the Q factor of a high-frequency resonance circuit which behavior is observed when the shield interval is varied, when the high-frequency resonance circuit uses a frequency of 1,200 MHz, its Q factor is (1) about 38 if the shield interval (corresponding to the thickness d, described previously) is 0.6 mm, (2) about 34 if the shield interval is 0.5 mm and (3) about 26 if the shield interval is 0.4 mm. This high-frequency resonance circuit is constructed similarly to the laminated band-pass filter 500 described previously (see FIG. 5), and the strip line conductors are provided substantially in the center of the band-pass filter in the thickness direction thereof.